When an article to be processed is to be coated, it is commonly subjected to surface treatment from the viewpoint of ensuring the corrosion resistance and adhesiveness of a coating film. In particular, when a metal (metal material, metal structure) is to be coated, the metal surface is subjected to chemical conversion treatment (surface treatment) for chemically forming a chemical conversion coating film.
An example of the chemical conversion treatment is chromate conversion treatment with a chromate containing composition; however, adverse effects of chromium have been noted. In recent years, a zinc phosphate treatment agent (zinc phosphate treatment) has been widely used as a chromium-free treatment agent (surface treatment agent, chemical conversion treatment agent) (see, for example, Patent Document 1).
However, the zinc phosphate treatment agent is highly reactive due to the high content of metal ions and acids that it contains, hence this treatment agent offers an unfavorable impact on both the cost and the workability during drainage treatment. In addition, metal surface treatment with the zinc phosphate treatment agent involves the generation and sedimentation of water-insoluble salts. Such precipitates are generally referred to as sludge, and the removal and disposal of sludge generates an additional undesirable cost. Furthermore, the use of phosphate ions is not preferable because they may affect the environment through eutrophication, and the treatment of phosphate ion effluent requires considerable labor. Moreover, metal surface treatment with the zinc phosphate treatment agent requires surface adjustment, which can prolong the total treatment process.
In addition to the zinc phosphate treatment agent and chromate conversion treatment agent, a chemical conversion treatment agent containing a zirconium compound is known (see, for example, Patent Document 2). The chemical conversion treatment agent, including a zirconium compound, contains less metal ions and acids, and hence is not as reactive. This offers a favorable cost advantage and improved workability during drainage treatment. Such a chemical conversion treatment agent is also superior to the above-described zinc phosphate treatment agent in regard to the inhibition of sludge generation.
However, the chemical conversion coating film formed with the treatment agent, including a zirconium compound, may not have better adhesiveness to a coating film obtained by cationic electro-deposition or the like, compared to that obtained with the use of a zinc phosphate treatment agent. Therefore, the treatment agent, including a zirconium compound, has been combined with phosphate ions or other components in order to improve its adhesive properties and corrosion resistance. However, the combination of phosphate ions can cause the above-described eutrophication.
Also provided is a chemical conversion treatment agent, which includes a zirconium compound and an amino group-containing silane coupling agent in order to improve the adhesiveness (for example, see Patent Document 3). Through the use of the chemical conversion treatment agent, zirconium serves as a film forming component of a chemical conversion coating film, and the amino group-containing silane coupling agent improves the adhesiveness between a chemical conversion coating film and a coating film by acting not only on the surface of the metal material, but also on the coating film formed after chemical conversion treatment.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. H10-204649    [Patent Document 2] Japanese Unexamined Patent Application Publication No. H7-310189    [Patent Document 3] Japanese Unexamined Patent Application Publication No. 2004-218070
However, under the present circumstances where sophisticated surface treatment techniques are required, it has been an objective to develop a zirconium-based metal surface treatment composition which offers improved base metal concealing properties, coating film adhesiveness, and corrosion resistance.
Higher storage stability of the metal surface treatment composition is also demanded because if a metal surface treatment composition to be repeatedly used has poor storage stability, its effectiveness will deteriorate in a short period after being put to use, which will result in the failure of the formation of a chemical conversion coating film capable of offering original base metal concealing properties, coating film adhesiveness, and corrosion resistance. In particular, metal surface treatment compositions for large metal materials, such as automobile bodies or parts, are used in oversized treatment baths, and thus, it has been an objective to extend the lifespan of these compositions.